When dispensing gasoline to the tanks of individual vehicles at a retail service station, the gasoline typically is pumped from large storage tanks, which are often located underground. Such storage tanks, in turn, are refilled periodically from tanker trucks, which receive gasoline at a central distribution site and deliver it to individual service stations where it is pumped from the truck into the underground storage tanks.
Gasoline storage tanks at retail service stations typically are vented to the atmosphere so that air can be drawn into the tank to displace liquid gasoline as it is pumped from the tank and into the gas tanks of vehicles. As a result, the head space within the storage tank, i.e. the space above the surface of liquid gasoline in the tank, is progressively filled as the tank is emptied with a mixture of oxygen, nitrogen, and water vapor from the atmosphere, and highly concentrated gasoline vapor, which evaporates from the surface of liquid gasoline within the tank.
Atmospheric venting of gasoline storage tanks leads to numerous problems that have previously been difficult to address. One such problem stems from the tendency of some of the water vapor in the atmospheric gases to condense inside the storage tank. Since the resulting liquid water is more dense than gasoline, it tends to settle at the bottom of the tank creating a water/hydrocarbon interface between the collected water and the gasoline. It is well known that certain bacteria fungi tend to thrive at this interface. The activity of such bacteria fungi results in the formation of a sludge-like material at the bottom of the tank.
In the past, this biological sludge has presented no insurmountable problems because it was localized at the tank bottom. However, certain gasoline additives, such as ethanol or MTBE, that recently have been added to gasoline in an effort to minimize carbon monoxide formation and comply with more stringent reformulation specifications required by recent legislative activity arising out of the Clean Air Act. Such additives, however, have tended to dissolve this sludge into the gasoline. As a result, filters and transfer conduits often become clogged with the dissolved sludge. Also, the sludge material tends to impede the proper functioning of automatic level controllers used to monitor potential tank leaks and provide automatic delivery options for retailers. The unforeseen sludge problem caused by the collection of water in storage tanks in conjunction with modern gasoline additives is thus becoming a major obstacle for retailers.
Another somewhat unforeseen problem related to the relatively recent addition of water soluble additives to gasoline has been the rapid dilution of gasoline pump sealant mixtures through contact with the dissolved water in the gasoline at centralized terminals using carbon based recovery technologies. More specifically, liquid ring pumps commonly used at such centralized terminals typically utilize a sealing mixture comprised of about 20% water and about 80% glycol. When dissolved moisture in gasoline being pumped comes in contact with this mixture, the water concentration of the seal increases until the sealing properties of the water/glycol mixture are drastically reduced. This has always been a bit of a problem since some moisture has inevitably dissolved in the gasoline. However, recent field data suggests that the new gasoline additives can worsen the problem significantly by drastically increasing the amount of water or moisture that becomes dissolved in the gasoline. In addition, the polar nature of the ethanol and MTBE molecule in additives should result in carbon desorption difficulties of existing carbon--based systems if high vacuum levels cannot be achieved by the regeneration system. High vacuum levels are possible only with an effective water/glycol sealant mixture. If substantial carbon bed volume cannot be sufficiently desorbed, the residual material will greatly decrease carbon bed capacity and subsequently increase the carbon bed regeneration frequency, which in turn reduces carbon bed life due to pulverization of the carbon granules.
Thus, water vapor and the resulting collected liquid water in current systems of gasoline storage and distribution is becoming increasingly undesirable for the following reasons, among others:
(a) Modern gasoline additives tend to dissolve additional water into the gasoline and this increased moisture content accelerates the dilution of glycol/water sealant used in liquid ring pumps at centralized terminals. This results in increased operating expenses for terminal operators since the diluted mixture must be removed and discarded as hazardous waste and fresh sealant mixtures prepared.
(b) The interfacial boundary between collected water and gasoline supports microbial activity that leads to the formation of sludge. The sludge, in turn, is dissolved into the gasoline by modern additives and causes clogging of transfer conduits and in-line filters on dispensing equipment at retail outlets. The dissolved sludge also interferes with the operation of automatic level sensing equipment.
(c) Water vapor molecules tend to attach themselves to carbon sites along with other hydrocarbons. The carbon bed capacity is temporarily reduced since active sites are being occupied by unwanted water molecules instead desired hydrocarbon molecules.
It can thus be appreciated that at retail gasoline and centralized terminal distribution sites, the occurrence of water vapor and, consequently, liquid water in gasoline storage tanks has lead to ever increasing problems and expenses for retail gasoline and terminal operators.
In the past, the gases and vapors within the head space of gasoline storage tanks were simply re-vented into the atmosphere each time the storage tank was filled from a tanker truck. In recent years, however, environmental concerns have lead to requirements that head space vapors within gasoline storage tanks be recovered when the tanks are refilled to prevent introduction of the gasoline vapors into the atmosphere. Usually, such vapors are simply directed through a recovery conduit into the head space of the tanker truck as liquid gasoline from the tanker truck is pumped into the storage tank. When the tanker truck has depleted its load of liquid gasoline and is filled with head space vapors collected from the storage tanks that were serviced, the tanker truck returns to a central distribution terminal. Here, the concentrated vaporous mixture is retrieved from the tanker truck as the truck is loaded with fresh product.
In some instances, the retrieved vaporous mixture is simply burned to minimize the impact of its release into the atmosphere. However, combustion methods generate large quantities of carbon dioxide, which is an undesirable by-product noted for contributing to greenhouse effects in the atmosphere. In many instances, however, the vaporous mixture is processed to condense the gasoline vapor to liquid gasoline and thus recover the liquid gasoline from the mixture. The recovered liquid gasoline can then be redistributed to individual service stations for sale.
Various techniques are available for recovering the gasoline vapor from such mixtures. The dominant method of recovery in the U.S. employs a carbon based, dual bed adsorption system. Hydrocarbons are adsorbed onto carbon material, and are subsequently vacuum desorbed, and are finally absorbed into a recirculated liquid phase stream of gasoline. Mechanical refrigeration techniques typically involve cooling the mixture to a temperature below the condensation point of gasoline, whereby the gasoline vapor condenses into liquid gasoline which can be collected for redistribution. A cryogenic gasoline recovery system is illustrated and discussed in an article by A. H. Hall entitled "Operational Experience of the BOC Liquid Nitrogen Condensation Vapour Recovery Unit".
Although liquid gasoline recovery systems have been somewhat successful in recovering gasoline from vaporous mixtures, they nevertheless have been plagued with numerous problems and shortcomings. For example, vapor mixtures recovered from gasoline storage tanks commonly include high concentrations of water vapor and oxygen from the atmosphere. Since water vapor has a much higher condensation temperature than gasoline, it tends to condense out of the mixture long before condensation of gasoline begins to occur. As a consequence, prior art recovery systems typically include a pre-cooler wherein the mixture is pre-cooled to a temperature between the condensation points of water and gasoline in an attempt to condense the water out of the mixture. The precondensed water is then collected and drained from the system before the condensation of gasoline is commenced. In addition, carbon adsorption recovery systems suffer from reduced efficiency due to active molecular adsorption sites being occupied by water vapor instead of hydrocarbon material.
Even with such pre-cooling, some water vapor remains in the mixture. As a result, during further cooling of the mixture to condense the gasoline vapor, this water freezes and reduces the efficiency of the heat exchange operation. Also the resulting small ice crystals tend to destroy the pumps, seals, and valves of the chosen system. Furthermore, when the condensed gasoline returns to normal temperatures, the ice crystals melt and mix with the gasoline, thus reducing the quality of the gasoline condensate and requiring further gravity separation techniques. Also, the condensation of water vapor from the mixture requires energy, which otherwise might be used in condensing the gasoline vapor itself.
In addition to problems associated with water vapor in the mixture, oxygen in the mixture can also cause problems. When liquid nitrogen is used as a condensing coolant, for example, there is a risk that the oxygen within the mixture will undergo a phase change to its liquid state. Naturally, intimate contact between highly volatile gasoline and liquid oxygen can create an extremely dangerous explosive condition. In addition, the mere presence of oxygen gas in the initial vapor mixture creates a potential for explosion that must be seriously considered when designing tanker trucks and processing equipment.
Accordingly, a continuing and heretofore unaddressed need exists for a gasoline storage and distribution system that addresses and solves the aforementioned problems with current systems. A further need exists for a vapor recovery methodology wherein the above discussed problems associated with water vapor and oxygen in the mixture are eliminated, where energy is not wasted condensing water out of the mixture, and wherein high quality liquid gasoline is recovered from the mixture economically and with minimum system complexity. It is to the provision of such a methodology that the present invention is primarily directed.